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In view of the diversity of gas reservoir classification and the differences of characteristics of geology and development in different gas reservoirs, it is necessary to study the adaptability of gas reservoir classification. This paper analyzes and summarizes the current classification methods of gas reservoirs, studies the microscopic pore characteristics and seepage mechanism of different gas reservoirs. The commonalities and individualities of seepage laws in different gas reservoirs are determined, the key factors that determine the development dynamics and effects of gas reservoirs are proposed, and the gas reservoirs types are divided from the perspective of development. The results show that the development characteristics of gas reservoirs are determined by the seepage law, and the seepage law is determined by the microscopic pore characteristics. Therefore, from the perspective of development, the microscopic pore characteristics are the fundamental of gas reservoir classification. Gas reservoirs can be divided into four types: tight, low permeability, medium and high permeability, and fractured. This classification method can unify the types of gas reservoirs according to the seepage laws of different gas reservoirs, establish corresponding seepage models, evaluate productivity, and predict performance. It is of great significance for rational, efficient and scientific development of gas fields.

In order to develop fractured–porous water-bearing gas reservoirs efficiently, it is necessary to understand the change laws of water invasion performance. Based on the π theorem, this paper adopted the dimensionless parameter that can reflect the change characteristics of water invasion performance to design the physical simulation experimental scheme of water invasion performance. The full-diameter cores taken from the reservoir of one certain fractured–porous water-bearing gas reservoir in the central Sichuan Basin were selected to carry out physical simulation experiment of water invasion performance. Then, by virtue of numerical inversion, the experimental results were converted into evaluation parameters of water invasion performance in water-bearing gas reservoirs. Afterwards, the change laws of water invasion performance in fractured–porous water bearing gas reservoirs were investigated, and the method for calculating key water invasion performance evaluation parameters and water volume was established. Finally, water invasion performance analysis and aquifer size evaluation were conducted with the Xu 2 Member gas reservoir, Upper Triassic in the Zhongba gas field of the Sichuan Basin as an example. And the following research results are obtained. First, in fractured–porous gas reservoirs with edge and bottom water, water invasion and gas production occur simultaneously. Before water production, there is a linear relationship between bottom hole pressure and reserve degree of reserve recovery. After water production, water invasion speeds up and the water–gas ratio (WGR) rises rapidly and finally approaches to a stable WGR of higher level. Second, the bottom hole pressure drop rate before water production and the stable WGR are mainly affected by the water volume, and the degree of reserve recovery per unit pressure drop and the stable WGR can be used to evaluate the water volume. Third, a method for mutual interpretation and evaluation of the aquifer size and the reserve degree of reserve recovery per unit pressure drop before water production and the stable WGR of fractured–porous gas reservoirs with edge and bottom water is established. In conclusion, the water invasion performance curve of the gas reservoir agrees with the core simulation experimental results. Based on the known water volume of a gas reservoir, the stable reserve degree of reserve recovery per unit pressure drop before water production and the stable WGR can be predicted; and on the basis of the stable WGR, the water volume of a gas reservoir can be predicted. The prediction results are in line with the production performance, providing a theoretical support for water invasion performance prediction and overall water control of fractured–porous water-bearing gas reservoirs.

For the sake of improving the reliability of the method for well pattern density optimization of tight sandstone gas reservoir and effectively demonstrate the relationship between well pattern density and recovery efficiency, the well interference probability curve of the target study area in the Sulige Gas Field was plotted by establishing the calculation method for well interference probability. Then, a new method suitable for well pattern density optimization and recovery efficiency evaluation of tight sandstone gas reservoirs was established. Finally, this new method was applied in three encryption test areas of the Sulige Gas Field. And the following research results were obtained. First, the well interference probability of tight sandstone gas reservoir is heavily dependent on the well pattern density. With the increase of the well pattern density, the well interference probability presents a gradual increasing trend. And the well interference probability doesn't reach or approach to 1 until the well pattern density reaches a higher value. Second, in the three encryption test areas of the Sulige Gas Field, the economically optimal well pattern density is in the range of 2.6–3.1 wells/km2, and its corresponding recovery efficiency is 36.6–39.8% and corresponding well interference probability is 28–33%, while the economic limit well pattern density is in the range of 5.2–6.6 wells/km2, and its corresponding recovery efficiency is 46.8–49.8% and corresponding well interference probability is 83–89%. Third, the well interference probability corresponding to the economically optimal well pattern density of tight sandstone gas reservoir in the Sulige Gas Field is about 30%, and that corresponding to the economic limit well pattern density is about 85%. In conclusion, by virtue of this new method, the economically optimal well pattern density and the economic limit well pattern density and their corresponding recovery efficiencies can be calculated, so as to realize the well pattern optimization and recovery efficiency evaluation of tight sandstone gas reservoirs. The research results can provide theoretical support for the economic and efficient development of the Sulige Gas Field, and useful reference for the benefit development of the same type of gas reservoirs.

Tight sandstone gas reservoirs commonly contain water, so liquid loading often appears near wellbores, leading to production decline and even shutdown of gas wells. Therefore, the study on the change of water saturation near wellbores is of great significance to understanding the water production mechanisms of gas wells. In this paper, a set of physical simulation experiment procedures of identifying the change of water saturation near wellbores was designed according to the principle of radial well seepage of gas wells, and the production performance after vertical well fracturing in gas reservoirs was simulated by connecting tight cores with a diameter of 10.5 cm, 3.8 cm and 2.5 cm in series in a descending order of distance. According to the depressurizing production mode of gas wells, tubes with small diameters of 20, 30, 40 and 50 μm were used to simulate gas well tubing to control the gas production rate. And the change of water saturation near wellbore in the process of depletion production and its influencing factors were investigated. Finally, combined with actual data of production wells, the water saturation and water production of gas wells near wellbores and in different zones were calculated at the above four different small diameters of tubes and the changes thereof were also analyzed. The following results were obtained. First, each gas production rate corresponds to a critical water saturation. When the initial water saturation is lower than the critical value, the formation water flowing near the wellbore and in the middle zone can be carried out along with the production of gas and no liquid loading is formed. Second, when the initial water saturation is higher than the critical value, a large amount of formation water migrating from the far-wellbore zones accumulates near the wellbore, and thus liquid loading occurs at the bottom hole. Third, when the initial water saturation is equal to the critical value, the higher the gas production rate is, the more easily liquid loading tends to form near the wellbore. Fourth, for the same water saturation, water production increases and recovery factor decreases with the increase of gas production rate. In conclusion, the cumulative water production chart of a gas well generated by the physical simulation experiment method proposed in this paper agrees well with the water production behavior of the corresponding gas well. The research results are conducive to the effective prediction of gas well water production and can be used as guidance for the reasonable gas well water control.

A prerequisite to effective shale gas development is a complicated fracture network generated by extensive and massive fracturing, which is called SRV (stimulated reservoir volume) section. Accurate description of gas flow behaviors in such section is fundamental for productivity evaluation and production performance prediction of shale gas wells. The SRV section is composed of bedrocks with varying sizes and fracture networks, which exhibit different flow behaviors – gas diffusion in bedrocks and gas seepage in fractures. According to the porosity and permeability and the adsorption, diffusion and seepage features of bedrocks and fractures in a shale gas reservoir, the material balance equations were built for bedrocks and fractures respectively and the continuity equations of gas diffusion and seepage in the SRV section were derived. For easy calculation, the post-frac bedrock cube was simplified to be a sphere in line with the principle of volume consistency. Under the assumption of quasi-steady flow behavior at the cross section of the sphere, the gas channeling equation was derived based on the Fick's laws of diffusion and the density function of gas in bedrocks and fractures. The continuity equation was coupled with the channeling equation to effectively characterize the complicated gas flow behavior in the SRV section. The study results show that the gas diffusivity in bedrocks and the volume of bedrocks formed by volume fracturing (or the scale of fracturing) jointly determines the productivity and stable production period of a shale gas well. As per the actual calculation for the well field A in the Changning–Weiyuan Block in the Sichuan Basin, the matrix has low gas diffusivity – about 10−5 cm2/s and a large volume with an equivalent sphere radius of 6.2 m, hindering the gas channeling from bedrocks to fractures and thereby reducing the productivity of the shale gas well. It is concluded that larger scale of volume fracturing and higher fracture density in the SRV section are important guarantees for efficient development of shale gas reservoirs.